1. Field of the Invention
The present invention relates to dielectric filters and dielectric duplexers for use with high-frequency signals at hundreds of megahertz to a few gigahertz, and more particularly to polar dielectric filters having a sharp attenuation pole in pass characteristics and dielectric duplexers having such a filter incorporated therein.
2. Description of the Related Art
Various high-frequency filters are used in communications devices. Filters of reduced sizes and improved characteristics are especially required with prevalent use of portable telephones and like mobile communications devices. The conventional filters for use at high frequencies include dielectric filters comprising coaxial dielectric resonators.
Coaxial dielectric resonators comprise a main body formed by a dielectric member and having a through bore extending through opposite end faces of the body and a conductor layer formed over the outer peripheral surface of the main body and over the inner surface of the body defining the bore. Electromagnetic waves are caused to undergo resonance within the main body of the device. When one of the end faces is provided with a conductor layer, the main body provides a 1/4 wavelength resonator. When both end faces are provided with the conductor layer or neither of the end faces has the layer, the main body provides a 1/2 wavelength resonator.
With reference to an equivalent circuit shown in FIG. 7a, a plurality of coaxial dielectric resonators 1a, 1b, 1c can be assembled into a band-pass filter by capacitively coupling these resonators as arranged in parallel to provide interstage coupling capacitances C.sub.1, C.sub.2, and capacitively coupling external input and output terminals 40, 41 to the resonators 1a, 1c at opposite ends of the parallel arrangement to form input and output coupling capacitances C.sub.IN, C.sub.OUT.
The band-pass filter can be modified to a polar filter having sharp attenuation characteristics involving an attenuation pole as seen in FIG. 8 by providing a series resonance capacitance C.sub.3 for one of the coaxial dielectric resonators 1a, 1b, 1c as shown in FIG. 7b.
FIG. 9 shows a conventional polar filter having the equivalent circuit of FIG. 7b and comprising three coaxial dielectric resonators 1a, 1b, 1c mounted on a substrate 6. External input and output terminals 40, 41 are disposed on the substrate 6 for electric signals to be transmitted to or received from external devices. The inner conductor layers formed in the through bores of the resonators 1a, 1b, 1c are connected by metal terminals 9a, 9b, 9c to respective conductor patterns formed on the substrate 6.
The input terminal 40 is connected to the resonator 1a serving as an input stage by way of an input coupling capacitor 80. The input-stage resonator 1a is connected to the resonator 1b providing an intermediate stage via an interstage coupling capacitor 81 and series resonance capacitor 83. The intermediate-stage resonator 1b is connected to the resonator 1c serving as an output stage via the series resonance capacitor 83 and an interstage coupling capacitor 82. The output-stage resonator 1c is connected to the output terminal 41 via an output coupling capacitor 84.
The conventional filter described requires, in addition to the resonators 1a, 1b, 1c and the substrate 6, a large number of external parts including the capacitors 80, 81, 82, 83, 84 and the metal terminals 9a, 9b, 9c, and many assembling steps, and is consequently large-sized.
An object of the present invention is to provide a polar dielectric filter which comprises coaxial dielectric resonators each having an electrode capacitively coupled to the inner conductor layer thereof so as to reduce the number of external parts conventionally needed.